Aromatic amine compounds, particularly substituted aniline compounds, have been dinitrated to form dinitroaniline compounds which are useful as herbicides. The desired nitration takes place in the aromatic ring of the amine. It is known that nitric acid is a nitrating agent useful to nitrate an organic compound.
One problem with nitrating an aromatic amine is the protection of the amino group because nitration of primary aromatic amines yields unstable products. In the case of secondary aromatic amines, it is believed that the intermediate N-nitro and N-nitroso compounds are rearranged in acidic conditions to ring nitrated products. Since the amine compounds are susceptible to oxidation, it is often necessary to protect the amine group during nitration, for instance by converting the amine to its acyl derivative.
A second problem is that the nitration process can be slowed by the accumulation of water. The nitration process is usually carried out with nitric acid as the nitrating agent. Water is a byproduct, and the presence of water interferes with the nitration process. It is known in the industry to use a large excess of a nitric and sulfuric acid mixture, as the nitrating agent to minimize the adverse effects of water accumulation. This large excess is costly, and the partially depleted nitric acid-containing mixture is waste. The prior art also apparently addresses this problem by adding a water-immiscible solvent to allow partition of water formed during nitration from the reaction mixture into the aqueous acid phase. However, separation of reactants into two phases can result in operational problems.
One method of minimizing water interference is to add a water scavenger to the nitration mixture. U.S. Pat. No. 4,101,582 describes nitrating 4-chloro-o-xylene with fuming sulfuric acid and fuming nitric acid at a temperature between 10.degree. C. and 60.degree. C. This minimizes the water buildup by the reaction of water with excess SO.sub.3 in the fuming acid. A major disadvantage to this method is the formation on large quantities of waste acid that pose potential environmental problems.
A second method of dealing with water is to nitrate in multiple steps, wherein weaker nitric acid is used to protonate the aromatic amine and then stronger nitric acid is used to nitrate the salt of the aromatic amine. For example, U.S. Reissue Pat. No. 33,168 describes a process to dinitrate a substituted aniline compound in several discrete steps, the first of which is reacting the substituted aniline compound with dilute, i.e., 20%-50%, nitric acid in the presence of a liquid, water-immiscible organic solvent to form a partially nitrated salt. The diluted nitric acid from the first step is then removed and concentrated nitric acid is added to effect dinitration. The mononitrated product is then reacted with the concentrated nitric acid to form a dinitro product. Finally, the water-immiscible organic solvent, i.e., chlorinated hydrocarbons, is removed by vacuum. This process requires multiple discrete steps and separations and results in the contamination of waste streams by chlorinated hydrocarbons.
The prior art also describes nitration methods that require protection of the amino group, i.e. by acylation prior to nitration. The amino group may be protected by reacting the aromatic amine with acetic acid or acetic anhydride, performing the nitrating step, and then removing the acetyl group. For instance, U.S. Pat. No. 5,369,086 describes a process of producing 2-nitro-4-methyl-5-methoxyacetanilide by reacting 4-methyl-3-methoxyaniline with acetic acid and acetic anhydride. Then, nitric acid is added to the reaction mixture to give 2-nitro-4-methyl-5-methoxyacetanilide. The process uses approximately four times the stoichiometric quantity of nitric acid. Finally, this product is reacted with concentrated hydrochloric acid at reflux temperature to remove the acetyl group, giving 2-nitro-4-methyl-5-methoxyaniline. This is a costly additional step, and it creates waste.
The prior art has also described processes that do not protect the amine. For example, U.S. Pat. No. 3,726,923 describes a process of nitrating an aromatic amine to form a dinitro product in a single step. The reaction is carried out with a water-and acid-immiscible solvent that contains the organic reactant. Similarly, U.S. Pat. No. 4,136,117 describes a high yield process of nitrating N-alkylated secondary anilines in the 2,6 positions without protection of the amino group. The nitrating agent is a mixture of water, nitric acid, and sulfuric acid falling within certain concentration ranges. The N-alkyl-aniline is dissolved in an inert water-immiscible solvent, such as ethylene dichloride or other chlorinated hydrocarbon.
The above described methods generally produce substantial quantities of N-nitroso compounds, in part because large excesses of nitric acid are utilized. They also produce waste, including large quantities of waste acid and organic solvent waste.
What is needed is a process for dinitrating aromatic amine compounds wherein the reaction takes place in a single phase, and wherein large excesses of nitric acid are not required, and wherein sulfuric acid is not required, and wherein waste streams are not contaminated with chlorinated hydrocarbons and wherein the reaction is completed in a single step. What is also needed is a process wherein the generation of N-nitroso compounds is minimized.